Place the sugar and water in a saucepan over low heat and stir until the sugar is dissolved. Increase the heat and boil for 1 minute. Remove from heat and set aside to cool completely.
Process the watermelon in a food processor or blender to get 2 ½ cups (600ml) juice. Place in a large bowl, add the sugar syrup and the lemon juice and stir to combine.
Pour the mixture into an ice cream maker* and follow the manufacturer’s instructions until the sorbet is firm. Transfer sorbet to an airtight container and put in the freezer until firm, at least 4 hours.

*My ice cream maker would not hold all the mixture so I removed 2/3 cup of it prior to pouring into the machine